Power assisted steering gears are well known, particularly for motor vehicles as comprising a valve member mounted in a valve sleeve member and displaceable relative thereto, either axially or rotationally, in response to a steering input to adjust control ports between those members. Adjustment of the control ports serves to control flow of fluid under pressure from an appropriate pressure source to power assistance means which would be included in a steering system incorporating the gear and would usually be in the form of a double acting piston and cylinder or ram device. Usually relative displacement between the valve member and valve sleeve member is restricted or resiliently restrained by use of a torsion rod or spring component through which those members may be coupled together, particularly in the case where those members are rotationally displaceable relative to each other.
Conventionally the valve sleeve member has an external annular face slidably mounted in a sleeve cylinder of a valve housing which latter is usually integral with the steering gear housing and has at least three ports one of which is intended to be coupled to the fluid pressure supply while the other two are intended to be coupled to the power assistance means (for example to opposite chambers of a double acting piston and cylinder for the supply of pressure to and the relief of pressure from those piston chambers as appropriate to effect the power assistance). The control of fluid flow to and from the aforementioned ports is controlled by adjustment of the control ports between the valve and valve sleeve members; to achieve this it is conventional, as for example is disclosed in the power assisted steering gear shown in FIG. 1 of our U.K. Pat. No. 1,223,551, for the external annular face of the valve sleeve member to have at least three axially spaced annular recesses communicating one with each of the aforementioned three ports in the valve housing and through which fluid under pressure is supplied to or exhausted from the power assistance means of adjustment of the control ports. By this arrangement the three annular recesses will, from time to time, be subjected to fluid under pressure or exhaust depending upon the power assistance which is required. With this known arrangement the axially opposite and radially extending end faces of the valve sleeve member open to chambers in the valve housing which effectively form dead spaces within the gear and may be in constant communication with exhaust or reservoir to relieve pressure on the end faces and indeed these chambers may form part of the fluid return to reservoir following its flow through the control ports.
It is an important aspect of steering gear design that the dimensions of the unit should be reduced to a minimum which is practical and without loss of efficiency; in this way not only are the manufacturing costs of the gear likely to be reduced but the space which is necessary to accommodate the gear and steering system generally, are reduced which is regarded as of considerable importance in the layout of the technical systems for modern vehicles. With conventional power assisted steering gears the dimensions of the valve housing and its associated components are a considerable porportion of the overall dimensions of the gear and it is an object of the present invention to provide a means by which the space required for the valve can be reduced.